Allow inheritance between structs.

No subtyping, no interaction with traits. Partially addresses rust-lang#9912.

src/doc/rust.md

@@ -1169,6 +1169,21 @@ struct Cookie;
 let c = [Cookie, Cookie, Cookie, Cookie];
 ~~~~
 
+By using the `struct_inherit` feature gate, structures may use single inheritance. A Structure may only
+inherit from a single other structure, called the _super-struct_. The inheriting structure (sub-struct)
+acts as if all fields in the super-struct were present in the sub-struct. Fields declared in a sub-struct
+must not have the same name as any field in any (transitive) super-struct. All fields (both declared
+and inherited) must be specified in any initializers. Inheritance between structures does not give
+subtyping or coercion. The super-struct and sub-struct must be defined in the same crate.
+For example:
+
+~~~~ {.ignore}
+struct Sup { x: int }
+struct Sub : Sup { y: int }
+let s = Sub {x: 10, y: 11};
+let sx = s.x;
+~~~~
+
 ### Enumerations
 
 An _enumeration_ is a simultaneous definition of a nominal [enumerated type](#enumerated-types) as well as a set of *constructors*,

src/librustc/driver/driver.rs

@@ -276,7 +276,7 @@ pub fn phase_3_run_analysis_passes(sess: Session,
         last_private_map: last_private_map
     } =
         time(time_passes, "resolution", (), |_|
-             middle::resolve::resolve_crate(sess, lang_items, krate));
+             middle::resolve::resolve_crate(sess, lang_items, &ast_map, krate));
 
     let named_region_map = time(time_passes, "lifetime resolution", (),
                                 |_| middle::resolve_lifetime::krate(sess, krate));

src/librustc/front/config.rs

@@ -152,6 +152,7 @@ fn fold_struct(cx: &Context, def: &ast::StructDef) -> @ast::StructDef {
     @ast::StructDef {
         fields: fields.collect(),
         ctor_id: def.ctor_id,
+        super_struct: def.super_struct.clone(),
     }
 }
 

src/librustc/front/feature_gate.rs

@@ -52,6 +52,7 @@ static KNOWN_FEATURES: &'static [(&'static str, Status)] = &[
     ("simd", Active),
     ("default_type_params", Active),
     ("quote", Active),
+    ("struct_inherit", Active),
 
     // These are used to test this portion of the compiler, they don't actually
     // mean anything
@@ -187,11 +188,17 @@ impl Visitor<()> for Context {
                 }
             }
 
-            ast::ItemStruct(..) => {
+            ast::ItemStruct(struct_definition, _) => {
                 if attr::contains_name(i.attrs, "simd") {
                     self.gate_feature("simd", i.span,
                                       "SIMD types are experimental and possibly buggy");
                 }
+                match struct_definition.super_struct{
+                    Some(ref path) => self.gate_feature("struct_inherit", path.span,
+                                                    "struct inheritance is experimental \
+                                                     and possibly buggy"),
+                    None => {}
+                }
             }
 
             _ => {}

src/librustc/metadata/encoder.rs

@@ -1007,12 +1007,43 @@ fn encode_info_for_item(ecx: &EncodeContext,
                                  generics);
       }
       ItemStruct(struct_def, _) => {
+        // Create a list of all structs which compose this struct. That is, the
+        // reflexive, transitive closure over struct_def's super-struct.
+        let mut cur_struct = struct_def;
+        let mut structs: ~[@StructDef] = ~[];
+        loop {
+            structs.push(cur_struct);
+            match cur_struct.super_struct {
+                Some(t) => match t.node {
+                    ast::TyPath(_, _, path_id) => {
+                        let def_map = tcx.def_map.borrow();
+                        match def_map.get().find(&path_id) {
+                            Some(&DefStruct(def_id)) => {
+                                cur_struct = match tcx.map.find(def_id.node) {
+                                    Some(ast_map::NodeItem(i)) => {
+                                        match i.node {
+                                            ast::ItemStruct(struct_def, _) => struct_def,
+                                            _ => ecx.diag.handler().bug("Expected ItemStruct"),
+                                        }
+                                    },
+                                    _ => ecx.diag.handler().bug("Expected NodeItem"),
+                                };
+                            },
+                            _ => ecx.diag.handler().bug("Expected DefStruct"),
+                        }
+                    }
+                    _ => ecx.diag.handler().bug("Expected TyPath"),
+                },
+                None => break,
+            }
+        }
+
         /* First, encode the fields
            These come first because we need to write them to make
            the index, and the index needs to be in the item for the
            class itself */
-        let idx = encode_info_for_struct(ecx, ebml_w,
-                                         struct_def.fields, index);
+        let idx = structs.iter().fold(~[], |a: ~[entry<i64>], &s|
+            a + encode_info_for_struct(ecx, ebml_w, s.fields, index));
 
         /* Index the class*/
         add_to_index(item, ebml_w, index);
@@ -1032,7 +1063,10 @@ fn encode_info_for_item(ecx: &EncodeContext,
         /* Encode def_ids for each field and method
          for methods, write all the stuff get_trait_method
         needs to know*/
-        encode_struct_fields(ebml_w, struct_def);
+        structs.iter().advance(|&s| {
+            encode_struct_fields(ebml_w, s);
+            true
+        });
 
         (ecx.encode_inlined_item)(ecx, ebml_w, IIItemRef(item));